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DEVONIAN PERIOD

416-359 million years ago

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acanthostega skull

The skull of Acanthostega was more derived than those of rhipidistian fish but retained features from its fish ancestry.

The braincase of Acanthostega was similar to that of fish although the occipital region had begun to fuse in the region of the notochord (Clack, 2002).
Compared to earlier osteolepiforms, Acanthostega's snout was longer, its eyes were located farther back on its head and more medially, and the otic region was shorter. Acanthostega lost its lateral rostral bone around the nostril. Much of the region around the nostril was composed of cartilage rather than bone, perhaps a juvenile trait retained from its larval stage. Its choana was larger but similar to that of sarcopterygian fish. The hyomandibula was modified to become the stapes. Bones around the external nostril were lost as fish adapted to smelling in air. Acanthostega retained the lateral line system in the skull which had served its fish ancestors as a sensory system for hunting aquatic prey. Although the shoulder girdle had separated from the skull to create a neck, Acanthostega still possessed an anocleithrum like fish (the anocleithrum is one of the bones which had attached the shoulder to the skull in fish; Clack, 2002). The following illustration depicts the homology of the shoulder girdles of primtive fish and early tetrapods and contrasts the fish which possessed bones (in blue and red) to attach the shoulder to the skull.

comparison of shoulders